Question

Find the energy stored in a $60\text{cm}$ length of a laser beam operating at $4\text{mW}$.

• A. $8\times {10}^{-12}J$
• B. $4\times {10}^{-12}J$
• C. $8\times {10}^{-14}J$
• D. $4\times {10}^{-4}J$

Answer 1

The correct option is A $8\times {10}^{-12}J$

Given $L=60\text{cm}=60\times {10}^{-2}\text{m}$
Operating power is given as
$P=\frac{U}{t}$
$\Rightarrow U=P\times t$ ....(i)
So we can find ${}^{\prime }{t}^{\prime }$ by
$t=\frac{L}{c}=\frac{60\times {10}^{-2}}{3\times {10}^8}=2\times {10}^{-9}s$
Putting this in eq(i)
$U=4\times {10}^{-3}\times 2\times {10}^{-9}=8\times {10}^{-12}\text{J}$